Coupling networks are known in which the switching elements at each of the input line-output line connecting points are field effect transistors. Such an arrangement is described in DE-AS No. 19 22 382. Field effect transistors used as switches have a very high resistance in the blocked state which, at high frequencies, is limited mainly by the parasitic capacitance between the electrodes. To decrease the effect of these parasitic capacitances, the bulk terminal of each field effect transistor is, in known systems, connected to ground potential. However, under certain conditions, and particularly when the signals to be transmitted are high frequency signals such as, for example, video signals, difficulties arise when too many output lines are connected to one input line. Under these circumstances, a highly capacitive load is applied to the output of the input amplifiers.
To obviate the above-mentioned difficulty, a circuit is described in DE-AS No. 21 23 395, in which the bulk terminal of the field effect transistors is connected through a capacitor to ground potential when the field effect transistor is blocked, while it is ungrounded with respect to AC voltages when the field effect transistor is conductive. This is accomplished by use of an additional diode which is switched in accordance with the DC voltage applied to the gate of the FET. A compensation capacitor on the input side of each coupling point is switched to ground simultaneously with the switching of this additional diode. This capacitance is equivalent to the capacitance of the blocked FETs in a line, so that when a number of output lines are connected to one input amplifier, the capacitive loading on the latter remains substantially constant.
This allows the capacitive load on the input amplifiers to remain constant even when additional output lines are connected to the input line. However, it does not cause any decrease in the capacitance so that here, too, inductive disturbances may be transferred from the input line to the output line. Further, the arrangement is relatively expensive, since at each coupling point a number of additional switching elements must be supplied. This is of particular importance in larger coupling networks which then require a substantial number of additional components.